Make expr tree environment-controllable

MVM_JIT_EXPR_ENABLE needs to be set to nonempty value for the expr
tree compiler to run.

src/core/instance.h

@@ -187,6 +187,7 @@ struct MVMInstance {
 
     /* Flag for if jit is enabled */
     MVMint32 jit_enabled;
+    MVMint32 jit_expr_enabled;
 
     /* File for JIT logging */
     FILE *jit_log_fh;

src/jit/graph.c

@@ -2430,14 +2430,19 @@ static MVMint32 jgb_consume_bb(MVMThreadContext *tc, JitGraphBuilder *jgb,
     jgb->cur_ins = bb->first_ins;
     /* First try to create an expression tree */
     tree = MVM_jit_expr_tree_build(tc, jgb->graph, bb);
-    if (tree != NULL) {
+    if (tc->instance->jit_expr_enabled && tree != NULL) {
         MVMJitNode *node = MVM_spesh_alloc(tc, jgb->graph->sg, sizeof(MVMJitNode));
         node->type       = MVM_JIT_NODE_EXPR_TREE;
         node->u.tree     = tree;
         jg_append_node(jgb->graph, node);
         /* Log the tree */
         MVM_jit_log_expr_tree(tc, tree);
     } else {
+        if (tree != NULL) {
+            /* log the tree for fun and giggles */
+            MVM_jit_log_expr_tree(tc, tree);
+            MVM_jit_expr_tree_destroy(tc, tree);
+        }
         /* Otherwise, try to consume the basic block per instruction */
         while (jgb->cur_ins) {
             jgb_before_ins(tc, jgb, jgb->cur_bb, jgb->cur_ins);

src/moar.c

@@ -15,7 +15,7 @@ static void setup_std_handles(MVMThreadContext *tc);
 MVMInstance * MVM_vm_create_instance(void) {
     MVMInstance *instance;
     char *spesh_log, *spesh_nodelay, *spesh_disable, *spesh_inline_disable, *spesh_osr_disable;
-    char *jit_log, *jit_disable, *jit_bytecode_dir;
+    char *jit_log, *jit_expr_enable, *jit_disable, *jit_bytecode_dir;
     char *dynvar_log;
     int init_stat;
 
@@ -154,6 +154,10 @@ MVMInstance * MVM_vm_create_instance(void) {
     jit_disable = getenv("MVM_JIT_DISABLE");
     if (!jit_disable || strlen(jit_disable) == 0)
         instance->jit_enabled = 1;
+    jit_expr_enable = getenv("MVM_JIT_EXPR_ENABLE");
+    if (jit_expr_enable && strlen(jit_expr_enable) > 0)
+        instance->jit_expr_enabled = 1;
+
     jit_log = getenv("MVM_JIT_LOG");
     if (jit_log && strlen(jit_log))
         instance->jit_log_fh = fopen(jit_log, "w");